Dancing with the stars A bizarre stellar system made up of two dead stars 7000 light-years away has put Einstein's famous general theory of relativity under its most extreme test yet.

An international team of scientists discovered an unusually heavy neutron star that spins 25 times each second orbited by a white dwarf companion once every two and half hours.

Observations of the system, dubbed PSR J0348+0432, produced results consistent with Einstein's theory, scientists report in the journal Science.

Lead author, Dr John Antoniadis, of Germany's Max-Planck-Institute, and colleagues used radio and optical telescopes to search for subtle changes in the system.

"I was observing the system...looking for changes in the light emitted from the white dwarf caused by its motion around the pulsar," says Antoniadis.

"A quick on-the-spot analysis made me realise that the pulsar was quite a heavyweight. It is twice the mass of the Sun, making it the most massive neutron star that we know of and also an excellent laboratory for fundamental physics."

The large mass of the neutron star (the extremely dense dead corpse of a star far more massive than the Sun) and the closeness of its white-dwarf companion (the dead core of a Sun-like star), made the system an ideal candidate to test Einstein's general theory of relativity.

"We thought this system might be extreme enough to show a breakdown in general relativity, but instead, Einstein's predictions held up quite well," says Dr Paulo Freire, also of the Max Planck Institute.

The general theory of relativity, which Einstein published in 1915, explains gravity and the effects of mass on the fabric of space-time.

It expands on his special theory of relativity which explains light and the electro-magnetic force.

Antoniadis and colleagues were able to measure the decay of the two stars orbits as gravitational waves carried off energy from the system.

By very precisely measuring the time of arrival of the pulsar's radio pulses over a long period, they determined the rate of decay and the amount of gravitational radiation emitted.

Had competing theories been correct, the rate of orbital decay would have been different.

"Whereas relativity is not all that well tested because it's so hard, not having a black hole in your laboratory, which is probably a good thing!"

"It's only been tested to about two decimal places, [so] people have been rather unsure about how good relativity was. But this has added another decimal place to it and it still looks like Einstein's theory is working."

The findings also support the use of general relativity based templates for future gravity wave detectors.

Researchers using such instruments hope to detect the gravitational waves emitted by events such as dense pairs of neutron stars and black holes spiralling inward toward violent collisions.

Gravitational waves are extremely difficult to detect and even with the best instruments, physicists expect they will need to know the characteristics of the waves they seek, which will be buried in quantum noise from their detectors.

"The fact that this is giving more confirmation that relativity is working pretty well implies that the gravity waves will be out there at the strength people are predicting," says Francis.

"Therefore the next generation of gravity detectors should be able to find them."